Tax on Stock Gain and Loss

I'm confused with the tax on stock gain and loss and hope someone can shed some light.

I bought stock A at the highest price, then Dec 2022 I bought more at the lowest price, combined for a total of $10k

Jan 2023, I sold stock A for $10,100; gained $100. Will I be taxed with $100 or $10,100? And how does IRS know which one to tax me on?

For loss, what if I sold in Jan 2023 for $9000, loss $1000. Would i be taxed on $9000 i sold?

Will 1099 from my brokerage account has this info or are there additional forms I need for the gain and loss scenarios?

Any guidance is greatly appreciated.